1. Field of the Invention
The present invention relates to a semiconductor laser module with a grating fiber within which a Bragg grating is formed, and particularly relates to a module with an excellent high frequency performance.
2. Related Background Art
Wavelength division multiplex (WDM) systems for optical communication have been attractive because of abrupt increasing of information to be transmitted. Minimum interval to the neighboring wavelength in the WDM system is set to be 0.8 nm by ITU standard (International Telecommunication Unit). Distributed feedback laser diodes (DFB-LD) with an Mach-Zender type external modulator made of lithium niobate have been used as a light source for such a WDM system because of its sharp oscillation spectrum. In DFB-LD, the oscillation wavelength is determined by the Bragg grating formed within the laser chip.
A semiconductor optical amplifier with a grating fiber, which is called fiber grating laser, has been also attractive. The device comprises a semiconductor optical amplifier and a grating fiber within which a Bragg grating is formed in its core portion. The amplifier and the grating fiber are arranged so that the one facet of the amplifier and the Bragg grating make an optical resonator.
In the WDM system, the wavelength interval is set to be 0.8 nm as mentioned above, then the fiber grating laser is superior to the DFB-LD because the oscillation wavelength is primarily defined by the fiber grating and is adjusted independently of the semiconductor optical amplifier. However, the fiber grating laser has less performance than DFB-LD at high operating frequencies because the length of the optical resonator is longer than that of DFB-LD.
In the semiconductor optical amplifier driven with a high frequency signal, carriers in the active layer of the semiconductor is also affected by the signal. The variation of the carrier density causes the alteration of the refractive index in the active layer, the effective length of the optical resonator, and the oscillation wavelength responds dynamically to the signal frequency. Consequently, the width of oscillation spectrum (FWHM: Full width at Half Maximum) increase. This phenomenon is known as chirping. An optical source with a wider oscillation spectrum is inappropriate for the WDM system.